# std::erase, std::erase_if(std::forward_list)

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< cpp‎ | container‎ | forward list

Defined in header `<forward_list>` |  |
| (1) |
template< class T, class Alloc, class U > std::forward_list<T, Alloc>::size_type erase( std::forward_list<T, Alloc>& c, const U& value ); |  | (since C++20) (until C++26)
template< class T, class Alloc, class U = T > std::forward_list<T, Alloc>::size_type erase( std::forward_list<T, Alloc>& c, const U& value ); |  | (since C++26)
template< class T, class Alloc, class Pred > std::forward_list<T, Alloc>::size_type erase_if( std::forward_list<T, Alloc>& c, Pred pred ); | (2) | (since C++20)

1) Erases all elements that compare equal to value from the container. Equivalent to return c.remove_if([&](const auto& elem) -> bool { return elem == value; });.
2) Erases all elements that satisfy the predicate pred from the container. Equivalent to return c.remove_if(pred);.

### Parameters

c | - | container from which to erase
value | - | value to be removed
pred | - | unary predicate which returns ​true if the element should be erased. The expression pred(v) must be convertible to bool for every argument `v` of type (possibly const) `T`, regardless of value category, and must not modify `v`. Thus, a parameter type of T&is not allowed, nor is T unless for `T` a move is equivalent to a copy(since C++11). ​

### Return value

The number of erased elements.

### Complexity

Linear.

### Notes

Unlike std::forward_list::remove, `erase` accepts heterogeneous types and does not force a conversion to the container's value type before invoking the == operator.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithm (1)

### Example

Run this code

    #include <complex>
    #include <iostream>
    #include <numeric>
    #include <string_view>
    #include <forward_list>

    void println(std::string_view comment, const auto& c)
    {
        std::cout << comment << '[';
        bool first{true};
        for (const auto& x : c)
            std::cout << (first ? first = false, "" : ", ") << x;
        std::cout << "]\n";
    }

    int main()
    {
        std::forward_list<char> cnt(10);
        std::iota(cnt.begin(), cnt.end(), '0');
        println("Initially, cnt = ", cnt);

        std::erase(cnt, '3');
        println("After erase '3', cnt = ", cnt);

        auto erased = std::erase_if(cnt, [](char x) { return (x - '0') % 2 == 0; });
        println("After erase all even numbers, cnt = ", cnt);
        std::cout << "Erased even numbers: " << erased << '\n';

        std::forward_list<std::complex<double>> nums{{2, 2}, {4, 2}, {4, 8}, {4, 2}};
        #ifdef __cpp_lib_algorithm_default_value_type
            std::erase(nums, {4, 2});
        #else
            std::erase(nums, std::complex<double>{4, 2});
        #endif
        println("After erase {4, 2}, nums = ", nums);
    }

Output:

    Initially, cnt = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    After erase '3', cnt = [0, 1, 2, 4, 5, 6, 7, 8, 9]
    After erase all even numbers, cnt = [1, 5, 7, 9]
    Erased even numbers: 5
    After erase {4, 2}, nums = [(2,2), (4,8)]

### See also

removeremove_if | removes elements satisfying specific criteria (function template)
ranges::removeranges::remove_if (C++20)(C++20) | removes elements satisfying specific criteria (algorithm function object)
removeremove_if | removes elements satisfying specific criteria (public member function)

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